1. Field of the Invention
The present invention relates to a circuit board on which electronic devices such as semiconductor integrated circuit devices are to be mounted to constitute a computer system or the like.
2. Description of the Related Art
In accordance with an increase in demand for a miniaturized computer system which can operate at a high speed, the operation speed and the integration density of a semiconductor device to be mounted in the system has been increasing in an accelerated manner. Recently, a semiconductor device which has a clock frequency of 100 MHz and consumes a power of 30 W or more per chip, has been brought into practice. A circuit board on which such a semiconductor device is to be mounted must have a high heat-radiating property and a high speed signal-transmitting property, so that the superior characteristics of the device can reflect on the characteristics of the computer system. To improve a heat-radiating property, a high thermal conductivity is required, and to increase a signal transmitting speed, a low dielectric constant is required.
Concerning the heat-radiating property of the above two properties required for a circuit board, high thermal conductivity materials having a high thermal conductivity coefficient of about 30 to 3000 W.multidot.m.sup.-1 .multidot.K.sup.-1), such as AlN, BN, diamond, diamond-like carbon, BeO and SiC, are suitable for improving this property. However, the dielectric constant of the high thermal conductivity materials is higher, by 2.5 to 15 times, than that of low dielectric constant material described below. Thus, in the case of using the high thermal conductivity material, the signal transmitting speed is inevitably low. those of low dielectric constant materials.
On the other hand, concerning the signal transmitting speed, desirable materials for circuit board are the above-mentioned low dielectric constant materials, such as SiO.sub.2, polyimide, Teflon, which have low dielectric constants of about 3 to 3.8. Those materials are suitable for a circuit board, in order to increase the signal transmission speed. Further, a demand for lower dielectric constant materials has increased in accordance with the increase in signal transmission speed and the increase in size of the circuit board. Several materials having a dielectric constant of less than 3, which satisfy the demand, are known; however, none of them has satisfactory characteristics as a practical dielectric film, e.g., the resistivity, the moisture resistance, and the heat radiating property. In particular, the thermal conductivity of the low dielectric constant material is considerably low, as low as 1/3000 to 1/30 that of the high thermal conductivity material mentioned above. For example, an organic polymer film, such as polyimide or Teflon, has a thermal conductivity coefficient of only 1 W.multidot.-1.multidot.K.sup.-1. Therefore the low dielectric constant material is not suitable for a circuit board on which heat-generating devices are to be mounted to a high integration density.
As described above, it is substantially impossible at the present stage to achieve, using single material, both requirements for a circuit board, i.e., a high heat-radiating property and a high-speed signal transmission. Hence, it is proposed that the two requirements be satisfied by forming a composite circuit board constituted by a substrate formed of a material having a high thermal conductivity and a thin film formed of a low dielectric constant material coated thereon. In this case also, however, the thin film, on which electronic devices are directly mounted, must have not only a low dielectric constant but also a relatively high thermal conductivity. In addition, the composite circuit board has the following problems:
In general, a thin film of a dielectric material is formed on the surface of a substrate through a thin film forming process, such as vacuum deposition, sputtering, a cluster ion beam method, ion plating, ion mixing and CVD. The dielectric thin film formed through the thin film process has an internal residual stress, such as a tensile stress, depending on process conditions. Since the dielectric thin film having such a residual stress has low adhesion to the substrate, it is liable to be removed from the substrate. Similarly, when a wiring pattern is formed on the dielectric thin film having residual stress, as the adhesion between the film and the wiring pattern is low, delamination of the wiring pattern may arise. Further, when a dielectric film is formed on a substrate using a thick film process, a similar internal stress remains in the dielectric film due to contraction in a sintering process.